1. Field of the Invention
An object of the present invention is a method to remove artifacts in a nuclear magnetic resonance (NMR) imaging experiment. This method can be applied more particularly in medicine where it is sought to represent images of cross-sections of human bodies under examination. In these images, the method can be used primarily to remove disturbances resulting from so-called moving artifacts caused by movements of bodies or in the body.
2. Description of the Prior Art
In conventional practice, the representation of moving objects involves the taking of successive shots (the term "shot" being clearly used in a broad sense) uniformly spread out in time during the movement to be studied. By displaying these shots at a low rate, the movement of the object can be closely studied. By means of cameras, modern methods of television enable the application of this technique on condition that the cameras are sufficiently fast to acquire images which are very close to each other in time, and also on condition that these images are subsequently displayed by slowing down the projection rate. Unfortunately, these general techniques are not readily applicable to methods of imaging by nuclear magnetic resonance (NMR).
In fact, the acquisition of tomographic images with instrucments of this type is obtained only after a processing operation involving the reconstruction of the cross-section images. For example, the body particles located in the cross-section of a patient's body to be studies are subjected to an excitation sequence during experimentation and restitute, at the output, a signal containing an item of information which represents their nature. Unfortunately, all the particles of the cross-section re-emit a signal at the same time. With the known methods of image reconstruction, especially the so-called 2DFT method of reconstruction, the difficulty of this simultaneous response is circumvented by repeating the experiments a certain number of times and by changing the conditions of acquisition of the signal from one experiment to another, in the course of excitation-measurement conditions. Typically in NMR imaging, 256 successive experiments are performed over a total period with a minimum duration which is close to about of half a minute. Now the cycles of fluctuation of the human body, for example, in the region of the heart, are much faster. Consequently, the sequences are all acquired during a period which is longer than that of the cardiac pulse, for example. The images are fuzzy in the immediate region of the heart.
In order to overcome this disadvantage of motion, an initial solution consisted in the carrying out synchronizations. However, in NMR, the fastest methods of excitation known as steady-state free precession (SSFP), corresponding to free precession in a state of balance of magnetic moments of the particles, must be applied to a natural frequency. This natural frequency depends on the spin-spin relaxation time (designated as T.sub.2) of the magnetic moments of the particles to be studied. Of course, this natural frequency counters synchronization. There is in fact no reason why this natural frequency should be a multiple of the heart rate. If synchronization is adopted (and consequently if the idea of the SSFP mode is abandoned) and if, in addition, it is desired to produce an image which is said to be in T.sub.2 (in order to show the spin-spin relaxation time), it is then necessary to wait between each experiment for a period of time equal to three or four times the interval T.sub.1 (spin-lattice relaxation time) which is characteristic of the particles under study. In other words, the duration of each experiment is of the order of two seconds. The acquisition of 256 sequences then leads to a period of about 10 minutes for a single sychronized image during the cardiac cycle, taking into account the losses of time during synchronizations. However, physicians require about 32 images for a complete cardiac cycle in order to be able to gain a clear understanding of functioning of the heart. This results in a total examination time of five hours. A period of this length, however, is quite intolerable for patients. Furthermore, even assuming that patients could tolerate such a long period of examination, the cardiac cycle is not constant. The result is that synchronization of the other instants of the cycle with respect to a precise time of the cycle does not always correspond to the same state of the heart. A phase lead or phase lag with respect to a predetermined state of the heart depends on the acceleration or slowing-down of the heart rate with respect to a nominal estimated heart rate. As a result, the images exhibit artifacts.
In an alternative mode of excitation known as fast T.sub.2 with limited flip-over and with synchronization, it is possible to achieve a much higher speed since typically a fast T.sub.2 cycle lasts approximately 50 ms. Consequently, a second phase of the cardiac cycle can be acquired over a period substantially equal to twice this cycle. In imaging with normal resolution (256 sequences per cycle phase), this results in a total experiment time of 4 minutes. However, fast T.sub.2 imaging cannot be carried out simultaneously in a single slice since the NMR signal is excessivelly attenuated and there is a loss of contrast. Accordingly, a multislice technique is employed: during idle periods, measurements are made in other slices so as to let the signal revive. In practice, the experiment then lasts four times as long, namely 40 minutes, which is also quite excessive. Furthermore, there again arises the same problem of synchronization as that mentioned above. When the heart rate is not constant (over a period of 40 minutes!), the images also exhibit artifacts.
Another technique has been conceived by Michael L. Wood and R. Marc Henkelman (of the ONTARIO CANCER INSTITUTE AND DEPARTMENT OF MEDICAL BIOPHYSICS, UNIVERSITY OF TORONTO, TORONTO, CANADA). This technique has been published in Medical Physics, Volume 13, No. 6 of November/December, 1986. It essentially consists in determining an average value over a number of successive images in order to eliminate motion artifacts. However, this imaging technique, which is concerned with elimination of the effects of motion, is not wholly effective and certain unwanted elements remain in the image. The reason of the presence of these artifact is here also bound to the variation of the cardiac cycle during these experiments.
In the invention, these drawbacks are removed by using the advantages of a special method for image reconstruction, namely, the so-called 2DFT method. In this 2DFT method, at each sequence, an excitation is followed below the measurement of the NMR resulting signal, by a period during which a magnetic field gradient with a phase encoding function of the NMR signal is applied to the part of the body under examination. The value of this gradient develops as and when the sequences needed for the reconstruction according to this method are conducted. It can be shown that the choice of the pitch of the gradient is totally related to the dimension of the volume studied, measured along the direction of the axis on which this encoding gradient is applied. The effect of this phase encoding gradient is to cause a phase shift, in the received signal, of the contributions to the total NMR signal of the different parts of the body, as a function of their abscissae measured in the volume under examination along this axis of the encoding gradient. In practice, the significant resolution of the image obtained depends on the number of sequences undertaken, during which the phase encoding gradient varies step by step.
In the invention, it is decided to use a pitch of the variation of the phase encosing gradient which is a sub-multiple one of the normal one. That is, of the one which is totally related to the dimension of a useful studied volume. In a preferred example, which example is solely in order to give a clear picture, the variation pitch of the phase encoding gradient is eight times smaller than the normal pitch. The result of this is that each described image has, along the direction of this phase encoding gradient, a dimension which is eight times greater than the conventional image field. In other words, instead of examining what happens along a height, of 30 centimeters for example, along this phase encoding axis, a search is made to find out what happens on a height which is a multiple of 240 cm, where, finally, there is nothing. To obtain an image with the same resolution in the 30 useful centimeters, it then becomes necessary to describe this image, which is eight times higher, by eight times more sequences. In a practical example, this might result in a resolution of 256 lines in the useful image: it then becomes necessary to perform 8 to 256=2048 successive sequences.
It is then shown that, by Fourier processing, it is possible to reconstruct an image representing what is not stationary in the body under examination. This result is then obtained regardless of the development of the duration of the cycle of that part which may be cyclically in motion (the heart) in the examined body. The result of this is that the images of the stationary parts are now no longer dependent on the variation of the heart cycle. It shall be shown that, in practice, to obtain this image of stationary parts, it is necessary to reconstitute the significant image only partially (for example, for a Fourier transform with 256 times 8 points to obtain an image reduced to 256 useful lines).